[issue46981] Empty typing.Tuple

[Jelle Zijlstra]

Jelle Zijlstra  added the comment:

I tried out 3.11 on my pyanalyze type checker and got some failures because of 
this change, because my previous trick for distinguishing between Tuple and 
Tuple[()] failed.

3.10:

>>> from typing import get_args, Tuple
>>> get_args(Tuple[()])
((),)
>>> get_args(Tuple)
()


3.11:

>>> from typing import get_args, Tuple
>>> get_args(Tuple[()])
()
>>> get_args(Tuple)
()

However, the new behavior is more consistent: get_args(tuple[()]) always 
returned (). It's also easy enough to work around (just check `... is Tuple`).

I'll put a note in the What's New for 3.11 about this change.

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[issue46981] Empty typing.Tuple

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions: +Python 3.11

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[issue46981] Empty typing.Tuple

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 15df8f8d89a0e563bdd15e4cd6734298736a5a1d by Serhiy Storchaka in 
branch 'main':
bpo-46981: Remove typing._TypingEmpty (GH-31836)
https://github.com/python/cpython/commit/15df8f8d89a0e563bdd15e4cd6734298736a5a1d


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[issue46981] Empty typing.Tuple

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

If for repr(Tuple[()]), it is no longer needed.

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[issue46981] Empty typing.Tuple

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
keywords: +patch
pull_requests: +29934
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/31836

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[issue46981] Empty typing.Tuple

[Guido van Rossum]

Guido van Rossum  added the comment:

Alas, I have no idea. I don't even recall what copy_with() is for (it was 
apparently introduced in 3.7). Possibly vopy_with() is wrong here? I imaging 
some of this has to do with the special casing needed so that repr() of an 
empty Tuple type doesn't print "Tuple[]" (which IIRC it did, long ago).

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[issue46981] Empty typing.Tuple

[Jelle Zijlstra]

Change by Jelle Zijlstra :


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nosy: +JelleZijlstra

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[issue46981] Empty typing.Tuple

[Serhiy Storchaka]

New submission from Serhiy Storchaka :

There are two empty typing.Tuple. They have the same repr but are not equal.

>>> from typing import *
>>> t1 = Tuple[()]
>>> t2 = t1.copy_with(())
>>> t1
typing.Tuple[()]
>>> t2
typing.Tuple[()]
>>> t1 == t2
False
>>> t1.__args__
((),)
>>> t2.__args__
()

The only differences is that one has empty __args__, while other has __args__ 
containing an empty tuple. There is a code purposed to make __args__ containing 
an empty tuple in this case. What is the purpose?

It is not pure theoretical question. This affects unpacked TypeVarTuple 
substitution. With natural implementation Tuple[Unpack[Ts]][()] is not equal to 
Tuple[()] and I still have not figured which and where code should be added to 
handle this special case. It would be easier if such special case did not exist.

Built-in tuple does not have a special case:

>>> tuple[()].__args__
()

--
components: Library (Lib)
messages: 414892
nosy: gvanrossum, kj, serhiy.storchaka
priority: normal
severity: normal
status: open
title: Empty typing.Tuple

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